High power semiconductor laser diodes have become important components in the technology of optical communication, particularly because such laser diodes can be used for fiber pumping (amplification of optical signals) and other high power applications. In most cases features such as long lifetime, reliable and stable output, high output power, high electro-optic efficiency, and high beam quality are generally desirable. One key for the long-term reliability of modern high-power laser diodes depends on the stability of the laser facets cleaved to form the opposing mirrors of the laser cavity.
The physical degradation of laser facets is a complex reaction that can be driven by light, current, and heat, resulting in power degradation and, in severe cases, to catastrophic optical damage (COD) of the mirror surfaces themselves. A process developed by IBM and referred to as “E2 passivation” has been used to address these concerns and minimize the possibility of COD. As described in IBM's U.S. Pat. No. 5,063,173 entitled “Method for Mirror Passivation of Semiconductor Laser Diodes” issued to M. Gasser et al., the E2 process involves the deposition of a layer of silicon (or perhaps germanium or antimony) as a coating over the bare facet (mirror) surfaces. The presence of the coating functions as a passivation layer, preventing the diffusion of impurities capable of reacting with the mirror facet interface.
Today's laser diodes are operated at relatively high powers and these prior art passivation layers, as deposited, have been found to break down and allow for damage of the mirror surfaces to occur. Therefore, in order to obtain stable mirrors for infrared high power laser diodes, it has now become a standard practice to “condition” the passivation layer. As performed today, conditioning is an extremely time-consuming process that requires operating the laser diode at a reduced current level for a prolonged period of time (e.g., tens to hundreds of hours) so as to form a crystalline structure inside the as-deposited amorphous passivation layer, forming a stable interface between the passivation layer and the mirror facet. Besides the time period required for this conditioning process, it is necessarily performed on a device-by-device basis, further extending the time and expense of the fabrication process.